Disc drives are used for data storage in modern electronic products ranging from digital cameras to computers and network systems. Typically, a disc drive includes a mechanical portion, or head-disc assembly, and electronics in the form of a printed circuit board assembly mounted to an outer surface of the head-disc assembly. The printed circuit board assembly controls functions of the head-disc assembly and provides a communication interface between the disc drive and a host being serviced by the disc drive.
Typically, the head-disc assembly has a disc with a recording surface rotated at a constant speed by a spindle motor assembly and an actuator assembly positionably controlled by a closed-loop servo system. The actuator assembly supports a read/write head that writes data to and reads data from the recording surface. Disc drives using magneto resistive read/write heads typically use an inductive element, or writer, to write data to information tracks of the recording surface and a magneto resistive element, or reader, to read data from the information tracks during drive operations.
One type of data recorded to and read from the information tracks is servo data. Servo data, including a physical track identification portion (also referred to as a servo track number or physical track number), written to the recording surface define each specific physical track of a number of physical tracks written on the recording surface, and servo bursts, indicating fine position within a physical track. A servo track writer is traditionally used in writing a predetermined number of servo tracks to each recording surface during the manufacturing process. The servo tracks are used by the closed-loop servo system for controlling the position of the read/write head relative to the recording surface during disc drive operations.
High performance disc drives achieve areal bit densities in the range of several gigabits per square centimeter (Gbits/cm2). Higher recording densities can be achieved by increasing the number of bits per centimeter stored along each information track, and/or by increasing the number of tracks per centimeter written across each recording surface. Capacity increases gained through increasing the bits per centimeter stored on each track generally require improvements in the read/write channel electronics to enable data to be written to and subsequently read from the recording surface at a correspondingly higher frequency. Capacity increases gained by increasing the number of tracks per centimeter on each recording surface generally require improvements in servo control systems, which enable the read/write heads to be more precisely positioned relative to the information tracks.
Concentric servo tracks written across the recording surface of the disc is the intended result of a servo write process. Each of the concentric servo tracks should be a closed circle with its center coincident with the axis of rotation of the spindle motor. The concentric servo tracks should exhibit consistent track-to-track spacing relative to each other across the surface of the disc and track closure, i.e., each concentric servo track should conclude at a same radius as it began. However, in practice, factors such as spindle motor vibration, arm resonance and servo writer push pin resonance disrupt the formation of circular servo tracks during the servo write process.
A resulting discontinuity of the servo track caused by those disruptions is referred to as a “track closure error” or as a “track tear servo defect condition” while a resulting inconsistent track-to-track spacing is referred to as “written-in track squeeze.” Track closure errors and written-in track squeeze are effects caused by a relative shift in position between the read/write head and the recording surface during the servo write process at a time in the process that the servo data is being written to the recording surface. Written-in track squeeze and track closure errors are each variants of track mis-registration. The amount of mis-registration of one information track has a direct bearing on the ability of the read element to read data stored on an adjacent information track. If the mis-registration of the first information track encroaches on a second and adjacent information track by a sufficient amount, erasure or a partial erasure of data previously written to the second and adjacent information track can occur during a write operation to the first information track. The presence of a mis-registration of a first information track relative to a second adjacent information track, sufficient to cause adjacent track erasure, is referred to as “track squeeze.”
Track closure errors are evidenced by a position error signal difference between a first written servo burst and a last written servo burst of the servo track and may lead to servo off-track failures during drive operations. Typically, track closure errors result from a cumulative effect of a plurality of disturbances of varying frequencies. One of the more prominent frequencies of the plurality of disturbances present is a cage frequency of the bearing of the spindle motor. The cage frequency of the spindle motor bearing is the dominant frequency component of most written-in repeatable run-out errors present in the disc drive and accounts for nearly one half of the total disturbance causing track closure errors.
An approach taken by disc drive manufacturers to improve servo control systems has been through the introduction of compensation methods for repeatable run-out errors. Repeatable run-out errors cause the servo track formed during the servo write process to be an irregular, generally circular shape rather than a desired substantially perfect circle, which negatively impact the alignment of the read/write head relative to track center of the data track during data transfer operations. Servo tracks that are an irregular, generally circular shape cause off track conditions of the servo bursts relative to the data track once the data tracks have been formed. Data tracks are formed during a drive level formatting process and are based on the previously written servo tracks. Absent correction for the irregularly shaped servo tracks of the previously written servo tracks, the data tracks formed during the formatting process would mirror the shape of the servo tracks, which would decrease data through put efficiency. Through incorporation of appropriate correction techniques during the format process, and the use the use of those correction techniques during data transfer operations, a generally, substantially circular data track can be produce during the formatting process and used during data transfer operations.
One such construction of repeatable run-out error compensation recently proposed in the art is exemplified by U.S. Pat. No. 6,069,764 issued to Morris et al. The Morris solution incorporates a transformation of a sequence of time domain repeatable run-out values into a sequence of frequency-domain repeatable run-out values, dividing the frequency-domain repeatable run-out values by measured transfer functions of the servo system at selected frequencies, then inversely transforming the resulting frequency-domain sequence of compensation values to produce a sequence of time domain compensation values and injecting the time domain sequence of compensation values into the servo loop to compensate for the repeatable run-out error. This method used to compensate repeatable run-out error is referred to as Zero Acceleration Path (ZAP). ZAP uses the position error signal generated from the servo burst written on the recording surface during the servo write process to determine the real repeatable run-out error and to generate correction factors
As track densities continue to increase, track widths decrease and track closure errors become more prominent because the magnitude of the track closure errors relative to the track width increases. For disc drives of common form factor and configuration, the underlying disturbances causing track closure errors remain substantially consistent and produce fairly repeatable displacements between the read/write heads and the associated recording surfaces of the read/write heads. Typically, two forms of disturbances contribute to a large portion of a track closure error. The first disturbance form includes repeatable run-out disturbances that have frequencies lying outside disturbance frequencies selected for frequency based compensation, and the second disturbance form includes non-repeatable run-out events. The contribution to a track closure error of neither of these two disturbance forms is resolved by an application of current compensation techniques. As such, challenges remain and a need persists for improved techniques of resolving track closure errors. It is to this and other features and advantages set forth herein that embodiments of the present invention are directed.